For most people, the last time David Cameron displayed anything approaching enthusiasm for the Green cause was during his brief, and perhaps ill-advised, husky-hugging expedition.

Now suddenly the Prime Minister waves the banner of a rival party — they must, he insists, be included in any pre-election TV debate.

It doesn’t take much digging to reveal the true reason for this sudden cross-party warmth.

David Cameron watched Nick Clegg get mauled on TV by Nigel Farage — twice — last year, and he knows the debates as planned give the UKIP leader an opportunity to do the same to him.

If only there was some way to stop them happening at all. And it turns out there is, by demanding the invite be widened.

Mr Cameron’s call to involve the Greens has united the leaders of Labour, the Liberal Democrats and UKIP, who sent him identical letters demanding he think again.

We talk as if these debates are a long-established part of British election campaigns, rather than something we did for the first time five years ago.

The prime minister is walking the same path as his predecessors, including John Major and Tony Blair, who found their own reasons to avoid them.

The last thing you want is to share a platform with your rivals — leaders of other parties elevated to the same status as a prime minister.

You would enter that hall with everything to lose. The only reason Gordon Brown took part was because by that point he had nothing to lose.

So for David Cameron, the Green Party is a convenient shield. Yes, the Greens have an MP, and have had for five years, while UKIP only entered parliament a few months ago.

But UKIP’s level of support, in a series of elections, is far higher.

His demand makes his refusal to take part appear, at least in part, principled. Something his rivals seek to undermine immediately through their barrage of letters.

There’s a deeper question beside statistics — when did the leader of one party, even the prime minister, get to choose who he debates with? That’s a matter for discussion between all political parties and the broadcasters.

And if the Greens were afforded that nationwide platform, why not the SNP?

So, will there be pre-election debates this year? In some form, probably. But it’s impossible for the BBC, Sky etc to “empty chair” Mr Cameron, as his rivals want them to do.

The rules covering election campaigns mean that, even if debates did go ahead without the prime minister, they’d have to find some way to give the Conservatives equal airtime. Outside the structure of a debate, other parties would object they were being given an unchallenged platform.

But away from TV, on YouTube or a newspaper website, those fairness rules don’t apply. And if the other party leaders were to join an online debate, and Mr Cameron stayed away, that would be a legitimate news story for broadcasters.

If the prime minister’s rivals want to portray him as scared, running away from a confrontation, that message would scream out from those news bulletins.

David Cameron’s gamble is the bad publicity from scuppering the debates is less damaging than the sight of him losing a debate to Nigel Farage, something he seems to believe is a distinct possibility.